Involute foil regenerator

ABSTRACT

A regenerator having a plurality of involute foils disposed in an annular gap between an inner cylindrical tube and an outer cylindrical tube. The involute shape of the foils provides uniform spacing throughout the entire regenerator and substantial surface area for fluid contact.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to thermal regenerators and moreparticularly to a thermal regenerator that uses thin, planar sheets ofmaterial of sufficient thermal conductivity to form the heat transfersurfaces of the regenerator.

[0003] 2. Description of the Related Art

[0004] Many devices, and Stirling cycle machines in particular, includea thermal regenerator to which thermal energy is transferred from aflowing fluid, and from which thermal energy is transferred to thefluid. Regenerators are normally made with large surface areastructures, such as wool, foils or spheres, made of metal, such asstainless steel.

[0005] In a Stirling cycle engine, for example, a working gas is movedbetween a warmer space and a cooler space by a reciprocating displacerto drive a reciprocating piston. The gas is heated during one part ofthe cycle, and cooled during another part. When the warm gas is beingtransported from the warmer space, it flows through a regenerator, andthermal energy is transferred to the regenerator by convection, i.e.,the impingement of heated gas molecules on the regenerator's surfaces.The regenerator is warmed and the gas is cooled when thermal energy istransferred to the regenerator as the gas flows through the regeneratorto the cooler space.

[0006] Once the gas has been cooled in the cooler space, it is drivenagain through the regenerator; ordinarily in the opposite direction aswhen the gas was driven from the warmer space. The cooler gas flowingthrough the regenerator is warmed by the same convection mechanism bywhich the gas warmed the regenerator: impingement of gas molecules onthe regenerator's surfaces. Regenerators therefore improve theefficiency of the Stirling cycle engine because the gas enters theheated end pre-warmed, and gas enters the cooler end pre-cooled. Ofcourse, regenerators improve the efficiency of many machines other thanStirling cycle machines.

[0007] In conventional regenerators, there must be a substantial amountof contact between the flowing fluid molecules and the surfaces of theregenerator in order for substantial heat transfer to occur. One type ofregenerator used in Stirling cycle machines uses a long thin strip ofmetal, such as stainless steel, that is wound up in a roll and placed ina chamber through which gas flows longitudinally of the roll. Each layerof the metal has a space or gap between it and the next adjacent layerfor fluid to pass through.

[0008] Even though it is desirable to have uniform spacing of the layersof a regenerator, it is often difficult, in practice, to achieve suchuniformity of spacing. A temperature differential between the heated endand the cooled end may cause buckling, which varies the gap sizes.Additionally, the flow of fluid through a wound regenerator cannotdistribute evenly radially, which can cause areas with substantiallymore flow to expand or contract the metal more than areas with lessflow. All of these problems result in high fluid flow rates throughlarger gaps, and low flow rates through smaller gaps. Non-uniform flowis disadvantageous, because large gaps permit some gas flowing throughthe regenerator to make poor contact with the surfaces with whichthermal transfer should take place. Furthermore, the pressure drop thatis critical to the class of machines referred to as free-piston machinesis often compromised with conventional regenerators, thereby resultingin unanticipated dynamic motion of the moving parts.

[0009] There is therefore a need for a regenerator that maintainssubstantially uniform spacing throughout the entire region of theregenerator through which fluid flows.

BRIEF SUMMARY OF THE INVENTION

[0010] The invention is a regenerator through which fluid can flow fortransferring thermal energy into and out of the fluid. The regeneratorcomprises an inner wall having a radially outwardly facing cylindricalsurface. An outer wall is spaced radially outwardly from the inner wall,and is substantially coaxial with the inner wall. The outer wall has aradially inwardly facing cylindrical surface. An annular gap is therebyformed between the inner wall and the outer wall. A plurality of foilsis disposed in the annular gap. The foils extend along substantialinvolutes of the radially outwardly facing cylindrical surface of theinner wall. Each foil has a first edge mounted to one of the cylindricalsurfaces and a second edge spaced from the first edge. The second edgeis near the other of said cylindrical surfaces, and is circumferentiallydisplaced from the first edge.

[0011] In a preferred embodiment, each foil mounts at its respectiveinner edge to the radially outwardly facing cylindrical surface of theinner wall, and extends toward, and seats against, the radially inwardlyfacing cylindrical surface of the outer wall. In a still more preferredembodiment, each foil has at least one spacer disposed between it andeach next adjacent foil. The spacers can be tabs or regions of the foildeformed toward the next adjacent foil in the shape of a cup or anyother shape.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0012]FIG. 1 is a schematic end view illustrating the preferredembodiment of the present invention.

[0013]FIG. 2 is an end view in section illustrating the preferredembodiment of the present invention.

[0014]FIG. 3 is a view in perspective illustrating the present inventionin an intermediate state of manufacture with the foils in asubstantially planar orientation and at substantial right anglesrelative to a wall to which they are mounted.

[0015]FIG. 4 is an end view illustrating the present invention in anintermediate state of manufacture.

[0016]FIG. 5 is a view in perspective illustrating the present inventionin an intermediate state of manufacture.

[0017]FIG. 6 is a schematic end view illustrating an alternativeembodiment of the present invention in an intermediate state ofmanufacture.

[0018]FIG. 7 is a view in perspective illustrating an alternativeembodiment of the present invention using rings extending throughapertures in the foils in which thicknesses are exaggerated to emphasizerelative surfaces.

[0019]FIG. 8 is a side view illustrating a foil with one embodiment ofspacers.

[0020]FIG. 9 is a view in perspective illustrating the foil of FIG. 8with its spacers in which thicknesses are exaggerated.

[0021]FIG. 10 is an end view illustrating the foil of FIG. 8 in anoperable position relative to other foils in which thicknesses areexaggerated.

[0022]FIG. 11 is a view in perspective illustrating an alternative foiland another embodiment of spacers in which thicknesses are exaggerated.

[0023]FIG. 12 is an end view illustrating the foil of FIG. 11 in anoperable position relative to other similar foils in which thicknessesare exaggerated.

[0024]FIG. 13 is a schematic side view illustrating the placement of aregenerator on a Stirling cycle machine.

[0025] In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific term so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose. For example, theword connected or term similar thereto are often used. They are notlimited to direct connection, but include connection through otherelements where such connection is recognized as being equivalent bythose skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The preferred embodiment of the regenerator 10 is shown in FIG.1, having an inner cylindrical wall 12 and an outer cylindrical wall 14.The inner wall is, in a preferred embodiment, a wall within which thedisplacer 13 of a Stirling cycle machine 15 reciprocates, as shown inFIG. 13. In a preferred embodiment, the outer wall 14 is coaxial withthe inner wall 12, and both the inner and outer walls are circularcylinders as shown in FIG. 2.

[0027] There is a gap formed between the radially outwardly facingcylindrical surface 22 of the inner wall 12 and the radially inwardlyfacing surface 24 of the outer wall 14. The gap is preferably annular,and extends a substantial portion, and preferably essentially theentirety, of the length of the inner and outer walls 12 and 14. In thecontemplated Stirling cycle machine 15, a fluid, such as the workinggas, flows through the annular gap 17 in a manner that will be apparentto those skilled in the Stirling cycle machine art and conventionalregenerators.

[0028] There are many foils 16 positioned in the annular gap between theinner and outer walls 12 and 14. The foils 16 are made of a material toand from which thermal energy is readily transferred, but which does nothave a high thermal conductivity that causes it to rapidly conduct thethermal energy to the surrounding structure. Stainless steel is apreferred material for the foils 16 used with engines (prime movers),and polyester or a similar plastic is preferred for coolers (heatpumps). The foils preferably have a length and width that issubstantially greater than their thickness. For example, a contemplatedfoil has a length of 60 mm, a width of 13.67 mm and a thickness of0.0254 mm. These dimensions are only exemplary, and it will beunderstood that the dimensions can vary significantly. For example, thewidth of a foil is determined by the distance across the annular gap,the angle of the attached edge, and other factors that cause the foil toform an involute.

[0029] Each of the foils 16 is mounted to the radially outwardly facingsurface 22 at its inner edge at spaced intervals of equal width, andeach extends along a path that is a substantial involute of the surface22 to contact the inwardly facing surface 24. The outer edges of thefoils can be welded, adhered or otherwise seated against the surface 24,but this is not required. The outer edges can be left free so that theyseat against the inwardly facing surface 24 and cause slight compressionof the foil regenerator structure. In this configuration, theregenerator conforms to accommodate the differential expansions thatoccur when using different materials for the foils and the walls 12 and14, such as plastic foils and metal walls.

[0030] By lying along an involute of the radially outwardly facingsurface 22, and being spaced at equal intervals around the cylindricalsurface 22, each foil 16 maintains a constant spacing relative to itsnearest neighbor along the entire length and width of each foil. Thus,there is a uniform spacing between each of the foils 16 at all radialand longitudinal positions, so that gas flowing through the annular gapdoes not have any larger pathways to flow preferentially through. Thisuniform flow prevents “hot spots”, and, likewise, “cold spots”, fromreducing the effect of the regenerator 10 on the efficiency of themachine to which it is mounted.

[0031] The regenerator 10 can be manufactured by one of several methods.In a preferred method, a substantially planar wall 32 has a plurality ofsubstantially parallel planar foils 36, each of which is attached at afoil edge along the wall's 32 major surface 42, preferably by welding,brazing or soldering when using metal foils and walls, or hot-melting,solvent bonding, ultrasonic welding or other plastic bonding techniquewhen the materials are plastic. Each foil's edge is mountedsubstantially perpendicular to the wall 32 equally spaced from eachadjacent foil by, for example, 0.115 mm for foils that are 0.0254 mmthick. Once all of the foils are attached, the structure has theappearance of a book when viewed along the planes of the foils 36 andthe wall 32 as shown in FIG. 3. Each of the foils is a “page” of the“book”, and the “spine” is the wall 32.

[0032] Once the foils 36 are all mounted to the wall 32, the wall 32 isdeformed, preferably by bending it around (away from the foils 36) toform a circular cylinder as shown in FIG. 4. The wall could be bent intoa rectangular cylinder or any other shape desired. The opposite edges ofthe wall 32 are connected together, such as by welding, to retain thepreviously planar wall 32 in the circular cylindrical shape to which itis bent. Each of the foils 36 retains its substantially planar shape,and is oriented radially of the wall 32.

[0033] The space between each of the foils 36 in the configuration shownin FIG. 4 is pie-shaped, because it increases in width as a function ofthe radial distance from the wall 32. If the regenerator were to beassembled in this configuration, the non-uniform gaps would permit mostof the gas to flow through the widest regions of the gaps between thefoils 36, at the greatest radial distance from the wall 32, because theresistance to fluid flow is least there.

[0034] Instead of assembling the regenerator when the foils are in theFIG. 4 configuration, the entire structure is next placed in adiameter-reducing device, such as a person's hand, a funnel-shaped tubeor another device, while at the same time rotating the wall 32 in onedirection. The outer edges of the foils 36 seat against the surface ofthe diameter-reducing device during the rotation of the wall 32, and dueto frictional resistance at the tips of the foils, all of the foils 36bend in one circumferential direction, such as clockwise as the radiallyinnermost edges rotate with the wall 32 and the radially outermost edgesstay seated against the device used to reduce the diameter of the foil36 and wall 32 combination. As all of the foils bend in the samecircumferential direction and the diameter of the diameter-reducingdevice decreases, the foils begin to form substantial involutes of thewall 32. When this occurs, the outer edges of the foils 36 are closer tothe wall 32, which permits the combination of the wall 32 and the foils36 to be inserted into an outer cylindrical wall against which the outeredges of the bent foils seat. The outer wall into which the wall 32 andfoils 36 is inserted has an inwardly facing cylindrical surface that iscloser to the radially outwardly facing surface 42 than the outer edgesof the foils 36 prior to bending. The final structure is structurallyidentical to that shown schematically in FIG. 1.

[0035] It also is possible to form a regenerator according to thepresent invention by first attaching a plurality of parallel foils to awall at an angle to the wall that approaches zero degrees. The wall isthen bent in the direction opposite that shown in FIG. 4 to form acylindrical outer wall, so that the foils extend inwardly of the wall.Then a tube is inserted within the outer wall after the foils are allrotated in the same circumferential direction and their inner edges areattached to the tube, which serves as the inner wall to form aregenerator. In this embodiment, the foils are attached at substantialright angles to the inner wall and curve outwardly along involutestoward the outer wall, intersecting the outer wall at the angle at whichthey were attached.

[0036] Another method of making the regenerator according to the presentinvention is to align a plurality of foils 46 parallel to one another ina “stack.” The spacers 48, which are preferably made of a similar oridentical material to the foils, but much shorter than the foils 46, areinterposed between each pair of foils 46 near the inner edges of thefoils 46. Next the stack of foils 46 is packed together in a tightrelationship with the spacers 48 all aligned near the inner edge of thefoils 46. Heat is then applied to the inner edge of the foils 46 and thespacers 48. The spacers 48 and foils 46 become hot enough to meltslightly at the inner edge, and then they are cooled, causingsolidification, which forms a thin wall 42 at the inner edge as shown inFIG. 6. The heat can be applied along parallel lines perpendicular tothe foils, and may be accompanied by a meltable rod, so as to weld thefoils and spacers together. Once the thin wall 42 is formed, it is thenbent into a cylinder, or bent around and attached to a cylinder, thefoils 46 are rotated circumferentially in the same direction and theentire device is placed in a cylindrical outer wall as in the methoddescribed in association with FIGS. 3 and 4.

[0037] Another alternative method of making a regenerator according tothe present invention is to insert one or more rings such as thestainless steel ring 50 shown in FIG. 7, through a plurality of alignedapertures formed near one edge of each of the foils 56. The ring 50 hasoverlapping ends to prevent foils from sliding off the ring 50. Spacers,such as shorter foils, can also be placed on the rings to space thefoils. Once all of the foils 56 are placed on the ring 50 by spreadingthe ends of the ring, the ring 50 springs closed and a circular cylindershaped tube is inserted within the ring 50 until the inwardly facingedges of the foils 56 seat against the radially outwardly facing surfaceof the tube wall. Then the foils can be attached to the tube, bentcircumferentially in the same direction, and then the entire structureis inserted into a second tube. Alternatively, the foils and spacers canbe heated to form a wall as in the embodiment described in associationwith FIG. 6.

[0038] Each of the foils of the regenerator of the instant invention canhave a spacer structure that mechanically maintains its spacing relativeto each next adjacent foil. In one embodiment shown in FIG. 8, a foil106 has tabs 110 that serve as spacers. Each tab 110 is formed bycutting the foil 106 along a U-shaped curve, and then pushing the freeend of the portion of the foil 106 that is within the U-shaped curve toone side along a path transverse to the plane that contains the foil 106as shown in FIGS. 9 and 10. In FIG. 10, the foil 106 is shown with itstabs 110 functioning as spacers by seating against a next adjacent foil104. The foil 108 has tabs 118 seating against the foil 106.

[0039] In an alternative embodiment shown in FIGS. 11 and 12, thespacers are bumps 120 formed in the foil 126. The bumps 120 can beformed by plastically deforming the foil 126, such as by forcing thefoil into a recess with a molded instrument, thereby stretching the foillocally. The tips of each of the bumps 120 seat against the nextadjacent foil 128, and the bumps 134 of the other adjacent foil 124 seatagainst the foil 126.

[0040] A regenerator made according to the instant invention may beplaced in an environment where a fluid, such as a liquid or a gas, flowsthrough it longitudinally in one direction during one part of a cycle,and then flows through it longitudinally in an opposite direction duringanother part of the cycle. In a preferred embodiment, the regenerator ismounted in a Stirling cycle machine with its inner and outer cylindricalwalls tack welded or otherwise rigidly connected to adjacent cylindricalstructures as shown in FIG. 13. The longitudinal ends of each foil aresupported against longitudinal and circumferential movement by tackwelding or by compressing metal wool (in the case of engines) or plasticfoam (in the case of heat pumps) between the longitudinal ends of thefoils and the adjacent structure. The wool or foam restricts the foils,and thereby resists any movement of the regenerator or its components asthe fluid is displaced rapidly first in one direction and then in theopposite direction. The wool or foam can serve some regeneratingpurpose, but most importantly acts as a mechanical stop to preventcircumferential movement of the foils or longitudinal movement of theentire structure or any component parts.

[0041] While certain preferred embodiments of the present invention havebeen disclosed in detail, it is to be understood that variousmodifications may be adopted without departing from the spirit of theinvention or scope of the following claims.

1. A regenerator through which fluid can flow for transferring thermalenergy into and out of the fluid, the regenerator comprising: a) aninner wall having a radially outwardly facing cylindrical surface; b) anouter wall substantially coaxial with the inner wall and spaced radiallyoutwardly therefrom forming an annular gap between the inner wall andthe outer wall, said outer wall having a radially inwardly facingcylindrical surface; c) a plurality of foils in the annular gapextending along substantial involutes of the radially outwardly facingcylindrical surface, each foil having a first edge mounted to one ofsaid cylindrical surfaces and a second edge spaced from the first edge,said second edge being near the other of said cylindrical surfaces andcircumferentially displaced from the first edge.
 2. The regenerator inaccordance with claim 1, wherein said first edge is an outer edge, andeach foil's respective outer edge seats against the radially inwardlyfacing cylindrical surface of the outer wall and each foil extendstoward the radially outwardly facing cylindrical surface of the innerwall.
 3. The regenerator in accordance with claim 1, wherein said firstedge is an inner edge, and each foil mounts at its respective inner edgeto the radially outwardly facing cylindrical surface of the inner walland each foil extends toward the radially inwardly facing cylindricalsurface of the outer wall.
 4. The regenerator in accordance with claim3, wherein the foils are mounted to the radially outwardly facingcylindrical surface of the inner wall at circumferentially spacedintervals.
 5. The regenerator in accordance with claim 4, wherein all ofthe circumferentially spaced intervals are substantially equal.
 6. Theregenerator in accordance with claim 5, wherein a plurality oflongitudinal gaps, each longitudinal gap being formed between one of thefoils and its respective next adjacent foil, extends from the inner wallto the outer wall.
 7. The regenerator in accordance with claim 6,wherein a gap width of each longitudinal gap is substantially the sameat all radial distances from the radially outwardly facing cylindricalsurface of the inner wall.
 8. The regenerator in accordance with claim7, further comprising a plurality of spacers, each spacer being mountedwithin one of said longitudinal gaps.
 9. The regenerator in accordancewith claim 8, wherein each spacer is a tab mounted to one of the foils.10. The regenerator in accordance with claim 9, wherein each tab isformed from a portion of each respective foil deformed toward the nextadjacent foil.
 11. The regenerator in accordance with claim 10, furthercomprising a curved incision in each foil surrounding at least part ofeach tab.
 12. The regenerator in accordance with claim 11, wherein thecylindrical surfaces are circular cylindrical surfaces.
 13. Aregenerator through which fluid can flow for transferring thermal energyinto and out of the fluid, the regenerator comprising: a) an inner wallhaving a radially outwardly facing cylindrical surface; b) an outer wallsubstantially coaxial with the inner wall and spaced radially outwardlytherefrom forming an annular gap between the inner wall and the outerwall, said outer wall having a radially inwardly facing cylindricalsurface; c) a plurality of foils in the annular gap extending alongsubstantial involutes of the radially outwardly facing cylindricalsurface, each foil having an inner edge mounted to said radiallyoutwardly facing cylindrical surface and an outer edge spaced from theinner edge, said outer edge being near the radially inwardly facingcylindrical surface and circumferentially displaced from the inner edge;and d) wherein the foils are mounted to the radially outwardly facingcylindrical surface of the inner wall at substantially equalcircumferentially spaced intervals.
 14. The regenerator in accordancewith claim 13, wherein a plurality of longitudinal gaps, eachlongitudinal gap being formed between one of the foils and itsrespective next adjacent foil, extends from the inner wall to the outerwall, and each longitudinal gap has a gap width that is substantiallythe same at all radial distances from the radially outwardly facingcylindrical surface of the inner wall.
 15. The regenerator in accordancewith claim 14, further comprising a plurality of spacers, each spacerbeing mounted within one of said longitudinal gaps.
 16. The regeneratorin accordance with claim 15, wherein each spacer is a tab mounted to oneof the foils.
 17. The regenerator in accordance with claim 16, whereineach tab is formed from a portion of each respective foil deformedtoward the next adjacent foil.
 18. The regenerator in accordance withclaim 17, further comprising a curved incision in each foil surroundingat least part of each tab.
 19. The regenerator in accordance with claim18, wherein the cylindrical surfaces comprise circular cylindricalsurfaces.
 20. A method of making a regenerator through which fluid canflow for transferring thermal energy into and out of the fluid, themethod comprising: a) disposing a plurality of foils transverse to athermally conductive wall, each of said foils having an inner edge nearthe wall and an opposing outer edge spaced from the wall, and the wallhaving a first edge and an opposing second edge; b) mounting the inneredge of each foil to the wall; c) bending the wall and mounting thefirst wall edge to the second wall edge, thereby forming an inner wallhaving a radially outwardly facing cylindrical surface to which thefoils are mounted; and d) disposing an outer wall substantially coaxialto the inner wall and spaced radially outwardly therefrom, said outerwall having a radially inwardly facing cylindrical surface, therebyforming an annular gap between the inner wall and the outer wall inwhich the foils are disposed extending along substantial involutes ofthe radially outwardly facing cylindrical surface of the inner wall,said outer edge of each foil being disposed near the radially inwardlyfacing cylindrical surface of the outer wall and circumferentiallydisplaced from the inner edge.
 21. A method of making a regeneratorthrough which fluid can flow for transferring thermal energy into andout of the fluid, the method comprising: a) disposing a plurality ofsubstantially planar foils substantially parallel to one another, eachof said foils having an inner edge and an opposing outer edge; b)inserting a spacer between each foil; c) heating the inner edge of thefoils and the spacers until they are bonded together, thereby forming awall at the inner edges of the foils, said wall having first and secondopposing edges; d) bending the wall and mounting the first wall edge tothe second wall edge, thereby forming an inner wall, the inner wallhaving a radially outwardly facing cylindrical surface to which thefoils are mounted; and e) disposing an outer wall substantially coaxialto the inner wall and spaced radially outwardly therefrom, said outerwall having a radially inwardly facing cylindrical surface, therebyforming an annular gap between the inner wall and the outer wall inwhich the foils are disposed extending along substantial involutes ofthe radially outwardly facing cylindrical surface of the inner wall,said outer edge of each foil being disposed near the radially inwardlyfacing cylindrical surface of the outer wall and circumferentiallydisplaced from the inner edge.
 22. A method of making a regeneratorthrough which fluid can flow for transferring thermal energy into andout of the fluid, the method comprising: a) disposing a plurality ofsubstantially planar foils substantially parallel to one another, eachof said foils having an inner edge and an opposing outer edge; b)forming at least one aperture in each foil, and aligning said apertures;c) inserting a ring through the aligned apertures in the foils; d)forming a wall at the inner edges of the foils, said wall having firstand second opposing edges; e) bending the wall and mounting the firstwall edge to the second wall edge, thereby forming an inner wall, theinner wall having a radially outwardly facing cylindrical surface towhich the foils are mounted; and f) disposing an outer wallsubstantially coaxial to the inner wall and spaced radially outwardlytherefrom, said outer wall having a radially inwardly facing cylindricalsurface, thereby forming an annular gap between the inner wall and theouter wall in which the foils are disposed extending along substantialinvolutes of the radially outwardly facing cylindrical surface of theinner wall, said outer edge of each foil being disposed near theradially inwardly facing cylindrical surface of the outer wall andcircumferentially displaced from the inner edge.